Afghan President Hamid Karzai traveled to western Afghanistan's Farah province Tuesday, about two weeks after U.S. airstrikes are alleged to have killed 140 civilians there.

Mr. Karzai, joined by U.S. Ambassador to Afghanistan Karl Eikenberry, met with grieving relatives and airstrike survivors at a mosque where more than 1,000 people had assembled.

Both men expressed their condolences to the crowd in the town of Farah, the capital of the province of the same name, and underscored the need to prevent civilian casualties.

Afghanistan's Defense Ministry says 140 civilians were killed in the U.S. airstrikes in Farah's Bala Buluk in early May, making it one of the deadliest incidents for civilians since U.S. forces began fighting the Taliban in 2001.

Eikenberry described the incident as "tragic," and he emphasized that the airstrikes were in response to a Taliban attack.

The U.S military has said a "number" of civilians were killed during fighting with Taliban militants. It has also accused the Taliban of using civilians as "human shields."

The U.S. military has started its own investigation into the incident.

It says the exact death toll cannot be determined because all the bodies were buried before investigators arrived.